The use of implantable probes or sensors to monitor the operation of organs within the human body has become considerably more common in recent years and, in particular, the continuous post-operative measurement of blood flow in vessels is frequently required in order to evaluate the results of medical procedures. A common and convenient type of probe for this purpose is of ultrasound Doppler form, such a probe being used as a blood flow velocity sensor. In the case of heart operations the ability to monitor post-operatively cardiac performance is required and this can be done by measuring the blood volume flow through the ascending aorta. The probe is located adjacent to the aorta during cardiac surgery and can be connected to monitoring equipment in the operating theatre. Subsequent to the operation the connecting leads can be unplugged and the patient moved to, say, an intensive care unit where similar monitoring equipment is available. The probe can then be connected again for continued monitoring over the required period of investigation.
To achieve accurate flow velocity measurements the Doppler probe must be attached firmly to the vessel under investigation. This is to ensure correct orientation of the probe and to ensure that the probe is not free to move relative to the wall of the vessel. However, in order to avoid a further operation to remove the probe from the vessel the probe must be extractable from outside the body of the patient without causing damage to the vessel or to the surrounding tissue. Known methods proposed for attaching the probe include suturing it to the vessel, or embedding a part of the probe directly in the vessel wall. The non-surgical removal is subsequently achieved by pulling the leads connected to the probe in order to withdraw it from the vessel and from the patient's body. Such removal can clearly be traumatic to the vessel and, because the probe is likely to present an irregular surface, damage can also be caused to other internal organs as it is withdrawn.
An implantable Doppler probe which is removable after use through a small opening in the patient's chest is described in WO-90/02519. The probe can be attached adjacent to the vessel or body under investigation by means of a suturing release feature.
The probe used for such medical purpose is naturally required to be completely sterile and this presents a number of problems, with no guarantee of complete sterility unless autoclaving used. Common practice is to discard the probe after removal and indeed in some jurisdictions it is illegal to reuse such a probe even if effective sterilisation is possible. As a disposable item the probes used are very expensive items due to the fact that they are largely handmade.
An accessory for an ultrasonic diagnostic probe is described in U.S. Pat. No. 4,593,699. As taught in this document, the probe and its connecting cable are sheathed in a removable sterile sleeve to obviate the need for sterilisation of the probe between uses. However, the operation of applying the sleeve to, or removing it from, the probe body must be carried out outside the patient's body.